1. Field of the Invention
This invention relates to microdensitometers, and more particularly to densitometers which are capable of submicron resolution.
2. Description of the Prior Art
An optical microdensitometer is an instrument which scans two dimensional arrays of information and provides information about the transmission at each point in the array. A film sample is an example of an array of information which is scanned by a microdensitometer.
An optical microdensitometer in its classical configuration consists of two microscopes facing each other and separated by that distance which will cause a transparent object placed between them to be in focus simultaneously in both of them. If the numerical apertures of the two microscopes are equal, the image formed by the second microscope will be found to be bordered by bright and dark bands of light that follow the contour of the object being viewed. This phenomenon is referred to as "ringing." Ringing results from the presence of increasing amounts of coherence in the light passing through the optics of the densitometer. Coherence increases as the numerical apertures of the microscopes decrease. This phenomenon degrades the performance of the system.
The standard solution to this problem is to mismatch the numerical apertures of the objective lenses in the two microscopes. Also, the second (output) microscope can be adjusted to examine a slightly smaller area than the area being illuminated by the first (input) microscope.
However, locating objective lenses with different numerical apertures becomes very difficult when the numerical apertures are rather large (e.g. on the order of 0.5 or larger). The fact is that the numerical aperture for a given power objective for high quality microscopes is very nearly the same for all manufacturers.
Iris type diaphragms cannot be used since diaphragms are not available for the small apertures typical of microscope objectives. Even if they were available, it would be very difficult to mount them properly at the aperture stop of the microscope objective.
However, if the source is imaged in the objective, the principal requirement of a condenser system will be fulfilled. That is, every point of the source will be illuminating any point in the object being examined. This type of illumination is used in projection systems and has not been seen to produce the "ringing" which is characteristic of coherent illumination systems.
Therefore, one objective of this invention is to provide a microdensitometer which is capable of submicron resolution. Another objective of this invention is to provide a simple apparatus for adjusting the coherence of the illumination in a microdensitometer.